Knowledge is broadly divided into anubhava or presentative cognition and smṛti or memory, i.e., representative cognition. Each of the two can be valid (yathārtha) or non-valid (ayathārtha). Valid presentative knowledge is called pramā. It is divided into perception, inference, comparison and testimony. Non-valid presentative knowledge (apramā) is divided into doubt (saṁśaya), error (bhrama or viparyyaya) and hypothetical argument (tarka). Thus valid presentative knowledge (pramā) is a definite or certain (asandigdha), faithful or unerring (yathārtha), and non-reproductive experience (anubhava)
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